You to be honest but at a 4 year old level. Believe me they can understand more than you can think. Keep it basic and also assure her that it is not dangerous to her or her parents - keeps need to know they are safe.
Say right now there is a virus out there - it is like a cold but it is very harmful to certain types of people...people that are very old or already sick. It is not something that would hurt a healthy little girl like you - it may make you a little sick but it would not make you harmful. In order to protect the older adults and care for people who really need it - we need to close many places so less people catch this virus. It might be hard for a little bit, but if we do this we will be helping others and soon we can go to all places we used to.
I had this happen to us on 9-11. We were on a Disney vacation with my 3 year old. We were supposed to fly home on the afternoon of 9-11. I had the foresight and also the fear that I did not want to fly back so I said lets rent a car and drive. Right after all this happen we called the car rental and reserved a car for the next day.
Any way in explaining this to my daughter - I said - there is a problem with the planes. No one can fly right now so we are going to drive home instead. It is a long drive so you will have to try to be as good as possible. We will stop over night at hotel and bring lots of books and coloring and games for you, but it is very important for you to be as patient and good in the car as you can.
It worked - I think she understood that something else was bigger but we made her feel safe and kept it as easy as possible.
You really do not need to go into too many details - it might confuse her and scare her. Try to come up with fun things you can do at home and just reassure her. Kids are much more understanding than adults. You probably know more than me on what level of detail and how much information would be better to address with her - each kid is different and some want more information and others just want some adult comfort.